This invention relates to electronic array probes for ultrasonic imaging, and in particular, to electronic array probes that comprise an array of transmitting and/or receiving electroacoustic transducers.
In the operation of electronic array probes, an ultrasonic beam is swept over a region of interest by electronic means which electronically generates time delays for acoustic radiation from each transducer. Thanks to this technique, the ultrasonic beam, which is generated by the acoustic contributions from all transducers, may be focused on one point, line or area of the region of interest, or the beam may be steered.
However, conventional two-dimensional array probes have the drawback of requiring a relatively large number of transducers in order to obtain sufficient resolution, resulting in the cable that connects the transducers to the controller having a large number of conductors, i.e., at least one conductor per transducer. This is a serious limitation, particularly for endocavitary or intraluminal probes, which are designed to be introduced in orifices or canals of the human or animal body, and are subjected to well-defined strict size restrictions. Therefore, both cable selection and probe installation are problematic and expensive, and also cause the probe to be more delicate to meet size restrictions that are barely compatible with the ideal size for endocavitary or intraluminal examinations.
Two different arrangements are known in the art to obviate this drawback. A first known arrangement provides for the use of a multiplexer and a cable having as many conductors as are needed for a subset of the total number of transducers, such that conductors are alternately switched to different subsets of transducers by the multiplexer. In addition to cost problems, the multiplexer is still a space-requiring electronic device, so therefore the problem is only partly solved. Also, while the multiplexing process allows the use of cables having a reduced number of conductors, i.e. smaller cables, it requires longer scanning times, as the whole transducer array is excited by way of a transducer subset exciting sequence, causing a longer beam forming time in addition to focusing or steering delays.
An alternative arrangement is known as a sparse array probe. Sparse arrays are two-dimensional arrays in which not all transducers are connected to the controller or not all the transducers are present. Hence, the number of conductors in the cable for connecting the probe to the control apparatus is actually reduced, but the acoustic signal dynamic range, i.e. the major to minor lobe ratio, is also reduced. Secondary or minor lobes are related to the number of transducers in the array. Therefore, sparse array probes typically have a large number of transducers, making them unsuitable for use in endocavitary probes.
A few examples of the foregoing arrangements are further detailed in U.S. Pat. Nos. 5,537,367 and 6,419,633.
U.S. Pat. No. 4,797,682 describes an endocavitary probe in which transducers are arranged in annular concentric bands and are adjacent and tangent to one another both within their respective annular bands and between annular bands, and in which transducers have identical extensions or active radiating areas in any one of the annular bands, and different extensions of active radiating areas from one band to the other. Transducers are described to have such sizes that the spacing between transducers, i.e. between the centers of the radiating surfaces of transducers, is irregular and does not account for the current restriction that requires spacing not to exceed half the wavelength of the acoustic pulse. While transducer arrays according to U.S. Pat. No. 4,797,682 provide encouraging results, they still have the drawback of requiring a large number of transducers having different sizes, whereby attention has to be paid during design to ensure that size differences actually work to minimize the number of transducers that, in the worst-case scan plane condition, have identical focusing delays on that plane. Therefore, the design of these types of probes is difficult and their construction requires a considerable number of transducer types having different radiating areas. More generally, there is no rule that allows for a reduction in the number of different transducers by providing a way to determine the number, pattern and type of the transducers required to form a predetermined array while obtaining the desired results.
Also, in the arrangement according to the above mentioned patent, transducer sizes vary from one annular band to the other; hence, the overall power delivered by each transducer varies according to its radiating area.
In the prior art, there are no phased array endocavitary probes that provide good dynamic range, i.e. sufficient major to minor lobe ratio, good resolution, and relatively low cost that allow them to be used with low-priced ultrasonic imaging apparatus.
From the technical point of view, the solution to this problem requires two contrasting requirements to be fulfilled. In fact, the attainment of high resolution and dynamic range, i.e. an optimized major to minor or side lobe ratio, requires the provision of a large number of transducers, and high dynamic range further requires spacing between transducer centers to be as small as possible, whereas probe size and cost reduction requires a reduction of the number of transducers, which affects resolution and dynamic range.
Therefore, the need arises for phased array ultrasonic probes which have a small size and a sufficient number of transducers such as to provide an optimized resolution and an optimized dynamic range, and which can be fabricated at such cost as to be able to be used with low-priced ultrasonic imaging apparatus, i.e., at very low costs.